> without them I presume the guards need to be coded manually elsewhere in the program.

That is my conclusion as well, reading the Oberon manual a SET seems just to be a normal set but for integers only, thus you have to manually check if your integer value is part of the set within the procedure itself, however that check will be a runtime check, not a compile time check if you had a proper enumeration type (I assume). And to my understanding Oberon doesn't have any error handling, so how exactly you would promote a out of range check to an error is unknown to me.

I guess that you would probably want to define constants to fill the set with, thus adding some more noise.

  CONST RED = 1;
  CONST GREEN = 2;
  CONST BLUE = 3;
  SET COLORS = (RED, GREEN, BLUE)

So you can use it a procedure call

  draw(RED);

Compare to Pascal enumeration

  type COLORS = (RED, GREEN, BLUE);

It feels like Wirth was somewhat obsessed with the idea of type extension with Oberon, it should be possible to import any module (program) and extend it without restrictions and enumerations created a problem how you would extend it.

If you have a color module defining the enumeration COLORS together with a bunch of procedures accepting COLORS, then in another custom module you want extend COLORS with the color YELLOW plus some YELLOW specific procedures.

But if you pass an extended enumeration E' to a procedure that accepts the base enumeration E, which should be legal similar how you can pass T' as T, the procedure would receive illegal values, in this case YELLOW. What you need to do instead is create your own CUSTOM_COLORS enumeration, manually include every color from COLORS and add YELLOW, and when calling procedures within the colors module type cast from CUSTOM_COLORS to COLORS enumeration. Perhaps this is the type explosion Wirth was referring to.

Out of range would still be problem if you extend a SET, but you avoid type casts.

This is how I interpret Wirth reluctance to enumerations (and subranges).

https://www.inf.ethz.ch/personal/wirth/Oberon/Oberon07.Repor...